1. Field of the Invention
The present invention relates to a readable and writable optical disc, and to a recording device and a reproducing device for the optical disk. More particularly, our invention relates to an optical disc for recording multimedia data including moving picture data, still image data, and audio data, and to a recording device and a reproducing device for this optical disc.
2. Description of the Related Art
Rewritable optical discs have for years had a maximum storage capacity of approximately 650 MB, but this has been changed by the development of phase change type DVD-RAM discs with a capacity of several gigabytes. Combined with the adoption of MPEG, and particularly MPEG-2, standards for encoding digital AV data, DVD-RAM is widely anticipated as a recording and reproducing medium with application in the AV industry as well as the computer industry. More specifically, DVD-RAM media are expected to replace magnetic tape as the storage medium of choice for AV recordings.
A. DVD-RAM
Increases in the storage density of rewritable optical disc media over the last few years has made it possible to use such media for applications ranging from storing computer data and recording audio data to recording image data, including movies.
The signal recording surface of a conventional optical disc is typically formatted with lands and grooves, one of which is used as a guide groove for signal recording and reproducing. The data signal is then recorded using only the land or the groove. With the advent of the land and groove recording method, however, it became possible to record signals to both the land and groove. This development approximately doubled the storage capacity of the disc.
Further development of a zone CLV (constant linear velocity) method simplified and made it easy to implement a CLV recording and reproducing technique, an effective means of further increasing the recording density.
A major topic left for future development is how to use such potentially high capacity optical disc media to record AV data containing image data to achieve new functions and performance far surpassing conventional AV products.
With the introduction of high capacity rewritable optical disc media, optical discs are widely expected to replace conventional tape media for recording and reproducing AV content. The transition from tape to disc recording media is also expected to greatly affect both the performance and functions of AV recording and reproducing products.
One of the greatest benefits of a transition to disc is a significant improvement in random access performance. While random access to tape content is possible, it generally takes on the order of minutes to rewind a full tape. This is several orders slower than the typical seek time of optical disc media, which is on the order of at most several ten milliseconds. Tape is therefore considered, for practical purposes, not to be a random access medium.
The random access capability of optical disc media has also made possible distributed, that is, noncontiguous, recording of AV data, which is not possible with conventional tape.
FIG. 34 is a block diagram of the drive device of a DVD recorder. As shown in FIG. 34, this DVD recorder comprises an optical pickup 11 for reading data from the disc 10, an ECC (error correction code) processor 12, track buffer 13, switch 14 for changing track buffer input/output; encoder 15, and decoder 16. An enlarged view of the disc 17 format is also shown.
As indicated by the disc 17 format, the smallest unit used for recording data to a DVD-RAM disc is the sector, which is 2 KB. Sixteen sectors are combined as one ECC block, to which the ECC processor 12 applies error correction coding.
The track buffer 13 is used for recording AV data at a variable bit rate in order to record AV data to a DVD-RAM disc more efficiently. While the read/write rate (Va) to a DVD-RAM disc is fixed, the bit rate (Vb) of the AV data is variable, based on the complexity of the AV data content (e.g., images if the AV data is video). The track buffer 13 is used to absorb this bit rate difference. This means that the track buffer 13 is unnecessary if the AV data bit rate is also fixed, as it is in the Video CD format.
This track buffer 13 can be even more effectively used by dispersed placement of the AV data on the disc. This is explained with reference to FIG. 35.
FIG. 35(a) shows the disc address space. If the AV data is recorded divided between contiguous area A1 between addresses a1 and a2, and contiguous area A2 between a3 and a4 as shown in FIG. 35(a), the AV data can be continuously reproduced from these non-contiguous areas A1 and A2 by supplying data accumulated in the track buffer 13 to the decoder while the optical head seeks from a2 to a3. This is shown in FIG. 35(b).
Once reading AV data starts from a1 at time t1, it is both input to the track buffer 13 and output from the track buffer 13 with data accumulating in the track buffer at the rate (Va−Vb), that is, the difference between the input rate Va to the track buffer and the output rate Vb from the track buffer. This continues to address a2 at time t2. Assuming that the data volume accumulated to the track buffer at this time is B(t2), data supply to the decoder can continue until the data B(t2) accumulated to the track buffer is depleted at time t3 at which reading resumes from address a3.
In other words, if it is assured that a certain volume of data ([a1, a2]) is read before a seek operation is performed, AV data can be continuously supplied to the decoder while the seek is in progress.
It should be noted that this example considers reading, that is, reproducing, data from DVD-RAM, but the same concept applies for writing or recording data to DVD-RAM.
It will thus be obvious that insofar as a specified amount of data is recorded continuously to DVD-RAM disc, continuous reproduction and recording is possible even if the AV data is noncontiguously recorded to the disc.
B. MPEG
A common AV data format is described next below.
As noted above, AV data is recorded to DVD-RAM media using the MPEG international standard, also known as ISO/IEC 13818.
Even though DVD-RAM discs have a large, plural gigabyte, capacity, this is still not sufficient for recording uncompressed digital AV data of any duration. A way to compress and record AV data is therefore necessary. This need was addressed by worldwide adoption of the MPEG (ISO/IEC 13818) standard for AV data compression. MPEG decoders (compression/decompression ICs) have also been realized with advances in IC devices. This has enabled the DVD recorder to handle MPEG compression and decompression internally.
MPEG signal processing is able to achieve high efficiency data compression chiefly as a result of the following two features.
First is that compression using a time correlation characteristic between frames (known as pictures in MPEG) is used in conjunction with conventional compression using a spatial frequency characteristic for moving picture data compression. Each video sequence of an MPEG video signal stream is divided into one or more groups of pictures, each group of pictures comprising one or more pictures of three different types: I-pictures (intraframe coded pictures), P-pictures (predictive-coded pictures, that is, intracoded with reference to a preceding picture), and B-pictures (bidirectionally predictive-coded pictures, that is, intraframe coded with reference to preceding and following pictures).
FIG. 36 shows the relationship between I, P, and B pictures. As shown in FIG. 36, P-pictures refer to temporally preceding I- or P-pictures in the sequence, while B-pictures refer to the first preceding and following I- or P-pictures. It should also be noted that because B-pictures reference an upcoming I- or P-picture, the display order of the pictures may not match the coding order of the pictures in the compressed data bitstream.
The second feature of MPEG coding is that code size is dynamically allocated by picture unit according to the complexity of the image. An MPEG decoder has an input buffer, and by accumulating data in this decoder buffer a large amount of code can be allocated to complex images that are difficult to compress.
Three types of audio coding are used for the audio portion of a DVD-RAM recording: MPEG audio with data compression, Dolby Digital® (also known as AC-3), and noncompressive linear pulse code modulation (LPCM). Both Dolby Digital® and LPCM are fixed bit rate coding methods, but MPEG audio coding can select from several compression rates on an audio frame basis, although audio compression is not as high as video stream compression.
The resulting compressed video and audio streams are multiplexed to a single stream using a method known as the MPEG system. FIG. 37 shows the organization of an MPEG system stream. As shown in FIG. 37, each 2 KB sector comprises a pack header 41, packet header 42, and payload 43. The MPEG system thus has a hierarchical structure comprising packs and packets. Each packet comprises a packet header 42 and payload 43. AV data is segmented from the beginning into blocks of an appropriate size for storage to the payload 43.
The packet header 42 records information referring to the AV data stored in the associated payload 43. More specifically, the packet header 42 contains a stream ID for identifying the data stored in the associated packet, and a decoding time stamp (DTS) and presentation time stamp (PTS) identifying the decoding time and presentation time of the data contained in the payload in 90 kHz precision. If the decoding and presentation are simultaneous, as in the case of audio data, the DTS can be omitted.
A pack is a unit of plural packets. In DVD-RAM, however, there is one pack for each packet, and each pack therefore comprises a pack header 41 and packet (containing a packet header 42 and payload to 43).
The pack header contains a system clock reference (SCR) expressing with 27 MHz precision the time at which the data contained in this pack is input to the decoder buffer.
An MPEG system stream thus comprised is recorded one pack to a sector (=2048 bytes) on DVD-RAM.
A decoder for decoding the above-noted MPEG system stream is described next below. FIG. 38 is a block diagram of an exemplary decoder model (P_STD) of an MPEG system stream decoder. Shown in FIG. 38 are the system time clock (STC) 51, that is, the internal reference clock for decoder operation; a demultiplexer 52 for decoding (demultiplexing) the system stream; video decoder input buffer (video buffer) 53; video decoder 54; re-ordering buffer 55 for temporarily storing I and P pictures to absorb the difference in the coding (data) sequence and presentation sequence that occurs between B pictures and I and P pictures; a switch 56 for adjusting the output order of the I, P, and B pictures buffered to the re-ordering buffer 55; an audio decoder input buffer (audio buffer) 57; and audio decoder 58.
This MPEG system decoder processes the above-noted MPEG system stream as follows.
When the time indicated by the STC 51 and the SCR written to the pack header match, the pack is input to the demultiplexer 52. The demultiplexer 52 then interprets the stream ID in the packet header, and passes the audio stream and video stream contained in the payload data to the appropriate decoder buffers. The PTS and DTS are also read from the packet header.
When the times indicated by the STC 51 and DTS match, the video decoder 54 reads and decodes the picture data from the video buffer 53. I and P pictures are stored to the re-ordering buffer 55 while B pictures are presented directly to screen. If the picture being decoded by the video decoder 54 is an I or P picture, the switch 56 switches to the re-ordering buffer 55 to output the previous I or P picture from the re-ordering buffer 55; if a B picture is decoded, the switch 56 switches to the video decoder 54.
Similarly to the video decoder 54, the audio decoder 58 reads and decodes one audio frame of data from the audio buffer 57 when the PTS matches the STC 51 (a DTS is not recorded for audio data).
An exemplary method of multiplexing an MPEG system stream is described next with reference to FIG. 39. Note that a sequence of video frames is shown in FIG. 39(a), the change in data storage to the video buffer is shown in FIG. 39(b), a typical MPEG system stream is shown in FIG. 39(c), and an audio signal is shown in FIG. 39(d). Each of FIGS. 39(a) to (d) are shown on a common time base (horizontal axis). The vertical axis in FIG. 39(b) indicates the amount of data stored to the video buffer. The bold line in this graph thus indicates the change over time in the buffered video data volume. The slope of this line is indicative of the video bit rate, and shows that data is input to the video buffer at a constant rate. The decrease in buffered data at regular intervals indicates the progression of data decoding. The intersection of the dotted line extension of the graphed line with the time base (horizontal axis) indicates the time at which video frame transfer to the video buffer begins.
MPEG encoding is described next using by way of example coding a complex image A in the video data stream. As shown in FIG. 39 (b), image A requires a large coding block, and data transfer to the video buffer must therefore begin from a time t1 before the image A decoding time. Note that the time from data input start time t1 to decoding is referred to as vbv_delay below. AV data is thus multiplexed to the position (time) of the shaded video pack.
Unlike video data, audio data does not require dynamic coding size control. It is therefore not necessary for audio data transfer to start at a similarly advanced time before decoding starts, and audio data is thus typically multiplexed only slightly before decoding starts. Video data is thus multiplexed to the MPEG system stream before the audio data.
It should be further noted that data can be accumulated to the buffer for a limited time in the MPEG system. More specifically, the MPEG system standard requires all data other than still image data be output to the decoder from the buffer within one second of being stored to the buffer. This means that there is at most a one second offset between video data and audio data multiplexing (or more precisely, the time required for video frame reordering).
It will also be obvious that while the MPEG system stream is described above with video data preceding the audio, the audio can theoretically precede the video. This type of stream can be purposely generated by using for the video data simple images to which a high compression rate can be applied, and transferring the audio data earlier than required. Even in this case, however, the audio can precede the video by at most one second due to the restrictions imposed by the MPEG standard.
Audio Stream Format and Reproduction
The format of the audio stream and a method for audio stream reproduction are described next below.
As described above, data is recorded and reproduced from a linear recording area in sequential access media such as magnetic tape. A typical method for recording an audio stream to a plurality of tracks on a single tape is described next below with reference to FIG. 41. In this example a maximum of two audio streams, shown as audio stream 1 and audio stream 2, can be recorded for a single video stream. In this example audio stream 1 is a single audio channel, generally known as a monaural audio channel, and audio stream 2 comprises two audio channels, such as a stereo audio signal or two monaural streams enabling a bilingual recording. It is also possible to record only one of these two audio streams (audio stream 1 or audio stream 2) or to record no audio stream. However, reducing the amount of audio recorded cannot be used as a means for increasing the video storage capacity of the tape. In other words, the audio stream recording area, i.e., the audio track space, is reserved exclusively for audio content and cannot be used for any other application even when no audio stream is actually recorded. The user can also select which of the two audio streams and channels to play, and the audio stream or channel selected by the user is reproduced simultaneously with the video.
DVD-RAM and other disc media, however, allow for more flexible audio stream recording and reproduction. The number of audio streams and channels recorded simultaneously with a video stream can be varied for the plurality of audio streams recorded to a disc.
FIG. 42 shows some of the ways in which the audio stream content can be varied with the video stream in a disc media. For example, AV stream 1 in FIG. 42(a) comprises one audio stream for the video stream, and the audio stream in this case has only one channel.
AV stream 2 in FIG. 42(b) similarly comprises one audio stream for the same video stream, but the audio stream in this case comprises two channels, i.e., main and sub audio channels. In this case the audio stream contains two selectively reproducible audio channels, a first audio channel containing the main audio (such as a first language), and a second audio channel containing the auxiliary audio data of the sub channel (such as a second language).
AV stream 3 in FIG. 42(c) comprises two audio streams for the video stream. In this case audio stream 1 is a single monaural channel while audio stream 2 contains two channels. The beginning of this audio stream 2 is recorded in stereo and then switches to dual monaural audio content. More specifically, audio stream 2 in this example comprises at least two of the following three audio content areas: a second (stereo) area containing first and second simultaneously reproduced audio channel data; a first (dual monaural) area containing first and second audio channels of which only one is selected and reproduced; and a third (monaural) area containing only one audio channel.
It will also be obvious that audio content is not limited to these stereo, dual monaural, and monaural types, and this audio stream 2 is simply illustrative of an audio stream containing a mix of different audio types. In the example shown in FIG. 42(c), audio stream 2 contains a stereo and a dual monaural area. Exemplary stereo content might be the commercials in a television broadcast while the dual monaural content contains separate audio streams of a bilingual broadcast in, for example, Japanese and English.
As noted above the relationship between video and audio streams on DVD-RAM and other disc media is flexible with the audio stream configuration being easily adapted according to the application and objective of the plural AV streams recorded to any same disc. It should be noted here that the AV stream configuration shown in FIG. 42 mimics a tape track configuration simply for ease of illustration and understanding. The actual AV stream configuration is a multiplexed bit stream of video stream data and one or more audio streams as shown in the MPEG system stream in FIG. 39(c).